1. Field of the invention
The present invention relates to a process for producing a toner used in a process by which a latent image is converted to a visible image.
2. Related Background Art
There is an image forming method in which an electrical latent image or magnetic latent image on a recording member is converted to a visible image by attracting electrodetective or magnetosensitive fine particles called a toner.
As electrophotography, which is a typical example thereof, various methods have been conventionally known, as disclosed, for example, in U.S. Pat. No. 2,297,691. In general, in this electrophotography, an electrostatic latent image is formed on a photosensitive member, utilizing a photoconductive material and according to various means, and subsequently the latent image is developed using a toner to form a toner image. Then the toner image is transferred to a transfer medium like paper if necessary, followed by fixing using a fixing means such as heat, pressure or solvent vapor. A copy is thus obtained.
Usually toners used for such purposes are produced by mixing and melting a coloring material comprised of a dye or pigment and a magnetic material in a thermoplastic resin and uniformly dispersing the coloring material, followed by pulverization and classification to obtain a toner having a given particle diameter. This method is relatively stable as a technique and can enjoy relatively easy control of the materials and process.
This method, however, is poor in energy efficiency, since the materials are melted together with a binder resin to mix and bind, and further the molten product is cooled, and then mechanically pulverized. Moreover, the resulting toner tends to have a broad range of particle size since fine particles are produced by the mechanical pulverization. Consequently as a step following the pulverization the toner must be classified, in order to obtain a fraction having the desired particle size distribution. This makes it difficult to obtain a high product yield. In order to solve such problems, a process in which the toner is produced by what is called suspension polymerization has been proposed as a new production process.
For example, Japanese Patent Publications No. 36-10231, No. 47-51830 and No. 51-14895 and Japanese Laid-Open Patent Application No. 53-17735, No. 53-17736 and No. 53-17737 disclose processes for producing a toner by the suspension polymerization. In the suspension polymerization, materials that are required to be contained in a toner as exemplified by a binder resin, a colorant such as a dye or pigment, a magnetic material, carbon black, a charge control agent and a release agent such as wax or silicone oil are uniformly dissolved or dispersed in a polymerizable monomer optionally together with a polymerization initiator and a dispersant to form a polymerizable composition, and this polymerizable composition is put in an aqueous continuous phase containing a dispersion stabilizer to form fine particles using a dispersion machine, then the particles being solidified by polymerization reaction so that toner particles with the desired particle diameters can be obtained in one step when the polymerization is completed.
This suspension polymerization enables omission of not only the melting step and pulverization step but also the subsequent classification step, and can be greatly effective for energy saving, time shortening, improvement of process yield, and cost reduction.
In suspension polymerization, including suspension polymerization for toners, increase in viscosity of its reaction system tends to occur as the polymerization proceeds, so that it becomes difficult for radicals and polymerizable monomers to move and hence polymerizable monomer components tend to be trapped in a large quantity in the polymer. In particular, for the suspension polymerization toners, more unreacted polymerizable monomers tend to remain because there is a large amount of components such as a dye or pigment (in particular, carbon black), charge control agent and magnetic material which may inhibit the polymerization. Not limiting to the polymerizable monomers, any component in the toner that may act as a solvent to the binder resin, may cause a lowering of fluidity of the toner making image quality poor, and also may cause a lowering of blocking resistance. Besides the toner performances, phenomena of deterioration of a photosensitive member other than the toner adhesion to the drum, such as memory ghost and unfocused images, may occur especially when an organic semiconductor is used as the photosensitive member. Besides such matters that concern the performances of products, there is a problem that the polymerizable monomer component evaporates during fixing to give off an offensive odor.
A means for decreasing the quantity of remaining polymerizable monomers may firstly be to improve polymerization conversion rate of the polymerizable monomers. As a method therefor, it is very effective to increase the amount of a polymerization initiator during polymerization, but the molecular weight distribution of the resulting toner shifts to a lower molecular weight making it impossible to obtain the desired molecular weight distribution (for styrene-acrylic types, molecular weights ranging from 10,000 to 50,000 give a good balance between fixing-starting temperature and fixing strength or toner strength). When plural kinds of polymerization initiators having different half-life are used so that a large amount of the polymerization initiators is present as a whole but radical species generated in the initiation phase is reduced, the shift to a low molecular weight and broadening the molecular weight distribution (this is important when heat-roller fixing is employed) can be effectively prohibited, but after all it can not overcome the problem of viscosity and can not be said to be fully satisfactory.
As methods for decreasing the viscosity of polymers and increasing the mobility of polymerizable monomers, followings can be contemplated: (i) add a solvent, (ii) add a plasticizer, (iii) add a chain transfer agent and (iv) raise temperature. Methods (i) and (ii), however, leave a problem in the toner when polymerization is completed. Method (iii) controls the formation of high molecular weight polymers that effect the viscosity without decreasing the quantity of radicals, but at present no satisfactory results has been obtained. In method (iv), polymers are melted by heat and at the same time thermal polymerization proceeds, where the polymerizable monomers can be more effectively consumed when a polymerization initiator which decomposes at a high temperature to produce radical species is present. This method, however, has difficulties in stabilizing dispersion and preventing agglomeration of polymerization toners.
Now, other than the consumption of polymerizable monomers by increasing the degree of polymerization, one may contemplate to collect polymerizable monomer vapor from a suspension to expell the polymerizable monomers remaining in the toner from the toner system, thereby to decrease the quantity of remaining polymerizable monomers. This, however, takes a very long time since this means the diffusion of an organic solvent through water. If the suspension system is vigorously stirred to increase the diffusion area in order to shorten time, air involved into the system causes bubbles, and toner particles adhered to the bubbles may come up to the surface of the suspension. Hence, there is a danger of producing a faulty toner because of agglomeration of toner particles and changes in polymerization conditions.
As a method of shortening the process time, Japanese Patent Application Laid-open No. 1-70765 discloses a method for producing a resin for a toner, in which, after suspension polymerization, the system is heated at a temperature higher than the Tg of the resulting resin to evaporate water in a quantity of 5% to 50% by weight based on the quantity of water at the time of completion of polymerization. This method certainly makes it possible to reduce the polymerizable monomers remaining in the resin in a short time, but consumes a large amount of energy. For the production of the polymerization toner, it is required a strict control of particle size, different from the resin for a toner to be obtained in the Japanese Laid-Open Patent Application No. 1-70765, in view of the prevention of agglomeration of particles. Thus, it is difficult to employ this method as it is.
As another method of shortening the production time, Japanese Laid-Open Patent Application No. 1-303450 discloses a method in which a polymerization product obtained by suspension polymerization is immersed and stirred during polymerization in an organic solvent capable of dissolving monomer components but not dissolving polymer components, and thereafter the polymerization product is collected from the solvent, followed by drying. This method, however, has a disadvantage that any component soluble in the organic solvent can not be added to the toner.